CHAPTER 16 The Citric Acid Cycle

– Cellular respiration– Conversion of pyruvate to activated acetate – Reactions of the citric acid cycle– Regulation of the citric acid cycle– Conversion of acetate to carbohydrate precursors

in the glyoxylate cycle

Key topics:

Only a Small Amount of Energy Available in Glucose is Captured in

Glycolysis

2G’° = -146 kJ/mol

Glycolysis

Full oxidation (+ 6 O2)

G’° = -2,840 kJ/mol6 CO2 + 6 H2O

GLUCOSE

Cellular Respiration

• process in which cells consume O2 and produce CO2

• provides more energy (ATP) from glucose than glycolysis

• also captures energy stored in lipids and amino acids

• evolutionary origin: developed about 2.5 billion years ago

• used by animals, plants, and many microorganisms

• occurs in three major stages:

- acetyl CoA production

- acetyl CoA oxidation

- electron transfer and oxidative phosphorylation

Respiration: Stage 1

Generates some:ATP, NADH, FADH2

Respiration: Stage 2

Generates more NADH, FADH2 and one GTP

Respiration: Stage 3

Makes lots of ATP

In Eukaryotes, Citric Acid Cycle Occurs in Mitochondria

• Glycolysis occurs in the cytoplasm• Citric acid cycle occurs in the mitochondrial matrix†

• Oxidative phosphorylation occurs in the inner membrane

† Except succinate dehydrogenase, which is located in the inner membrane

Conversion of Pyruvate to Acetyl-CoA

• net reaction: oxidative decarboxylation of pyruvate

• acetyl-CoA can enter the citric acid cycle and

yield energy

• acetyl-CoA can be used to synthesize storage

lipids

• requires five coenzymes

• catalyzed by the pyruvate decarboxylase complex

Pyruvate Dehydrogenase Complex (PDC)

• PDC is a large (Mr = 7.8 × 106 Da) multienzyme complex

- pyruvate dehydrogenase (E1)

- dihydrolipoyl transacetylase (E2)

- dihydrolipoyl dehydrogenase (E3)

• short distance between catalytic sites allows channeling

of substrates from one catalytic site to another

• channeling minimizes side reactions

• activity of the complex is subject to regulation (ATP)

Cryoelectronmicroscopy of PDC

• Samples are in near-native frozen hydrated state

• Low temperature protects biological specimens against radiation damage

• Electrons have smaller de Broglie wavelength and produce much higher resolution images than light

Three-dimensional Reconstruction from Cryo-EM data

Sequence of Events in Pyruvate Decarboxylation

• Step 1: Decarboxylation of pyruvate to an aldehyde

• Step 2: Oxidation of aldehyde to a carboxylic acid

• Step 3: Formation of acetyl CoA

• Step 4: Reoxidation of the lipoamide cofactor

• Step 5: Regeneration of the oxidized FAD cofactor

Chemistry of Oxidative Decarboxylation of Pyruvate

• NAD+ and CoA-SH are co-substrates

• TPP, lipoyllysine and FAD are prosthetic groups

Structure of CoA

• Recall that coenzymes or co-substrates are not a permanent part of the enzymes’ structure; they associate, fulfill a function, and dissociate

• The function of CoA is to accept and carry acetyl groups

Structure of Lipoyllysine

• Recall that prosthetic groups are strongly bound to the protein. In this case, the lipoic acid is covalently linked to the enzyme via a lysine residue.

The Citric Acid Cycle

Sequence of Events in the Citric Acid Cycle

• Step 1: C-C bond formation to make citrate

• Step 2: Isomerization via dehydration, followed by

hydration

• Steps 3-4: Oxidative decarboxylations to give 2

NADH

• Step 5: Substrate-level phosphorylation to give GTP

• Step 6: Dehydrogenation to give reduced FADH2

• Step 7: Hydration

• Step 8: Dehydrogenation to give NADH

Net Effect of the Citric Acid Cycle

Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2 H2O

2CO2 +3NADH + FADH2 + GTP + CoA + 3H+

• carbons of acetyl groups in acetyl-CoA are

oxidized to CO2

• electrons from this process reduce NAD+ and FAD

• one GTP is formed per cycle, this can be

converted to ATP

• intermediates in the cycle are not depleted

Direct and Indirect ATP Yield

Role of the Citric Acid Cycle in Anabolism

Regulation of the Citric Acid Cycle